How do I automatically send email messages to Junk
My inbox is loaded with unwanted messages some from dubious sources. I would like to selectively send them to the Junk folder before they appear in my inbox
That is not a Firefox issue.
Some e-mail providers have an option to filter incoming mail. Some also will send all mail to the junk / spam folder unless the user allows them to go to the inbox. Check with your e-mail provider.